Ethylene homopolymers and copolymers are a well-known class of olefin polymers from which various plastic products are produced. Such products include films, fibers, coatings, and molded articles, such as containers and consumer goods. The polymers used to make these articles are prepared from ethylene, optionally with one or more copolymerizable monomers. There are many types of polyethylene. For example, low density polyethylene (“LDPE”) is generally produced by free radical polymerization and consists of highly branched polymers with long and short chain branches distributed throughout the polymer. Due to its branched structure, LDPE generally is easy to process, i.e., it can be melt processed in high volumes at low energy input. However, films of LDPE have relatively low toughness, low puncture resistance, low tensile strength, and poor tear properties, compared to linear-low density polyethylene (“LLDPE”). Moreover, the cost to manufacture LDPE is relatively high because it is produced under high pressures (e.g., as high as 45,000 psi) and high temperatures. Most LDPE commercial processes have a relatively low ethylene conversion. As such, large amounts of unreacted ethylene must be recycled and repressurized, resulting in an inefficient process with a high energy cost.
A more economical process to produce polyethylene involves use of a coordination catalyst, such as a Ziegler-Natta catalyst, under low pressures. Conventional Ziegler-Natta catalysts are typically composed of many types of catalytic species, each having different metal oxidation states and different coordination environments with ligands. Examples of such heterogeneous systems are known and include metal halides activated by an organometallic co-catalyst, such as titanium chloride supported on magnesium chloride, activated with trialkyl aluminum. Because these systems contain more than one catalytic species, they possess polymerization sites with different activities and varying abilities to incorporate comonomer into a polymer chain. The consequence of such multi-site chemistry is a product with poor control of the polymer chain architecture, when compared to a neighboring chain. Moreover, differences in the individual catalyst site produce polymers of high molecular weight at some sites and low molecular weight at others, resulting in a polymer with a broad molecular weight distribution and a heterogeneous composition. Consequently, the molecular weight distribution of such polymers is fairly broad as indicated by Mw/Mn (also referred to as polydispersity index or “PDI” or “MWD”) Due to the heterogeneity of the composition, their mechanical and other properties are less desirable.
Recently, a new catalyst technology useful in the polymerization of olefins has been introduced. It is based on the chemistry of single-site homogeneous catalysts, including metallocenes which are organometallic compounds containing one or more cyclopentadienyl ligands attached to a metal, such as hafnium, titanium, vanadium, or zirconium. A co-catalyst, such as oligomeric methyl alumoxane, is often used to promote the catalytic activity of the catalyst. By varying the metal component and the substituents on the cyclopentadienyl ligand, a myriad of polymer products may be tailored with molecular weights ranging from about 200 to greater than 1,000,000 and molecular weight distributions from 1.0 to about 15. Typically, the molecular weight distribution of a metallocene catalyzed polymer is less than about 3, and such a polymer is considered as a narrow molecular weight distribution polymer.
The uniqueness of metallocene catalysts resides, in part, in the steric and electronic equivalence of each active catalyst molecule. Specifically, metallocenes are characterized as having a single, stable chemical site rather than a mixture of sites as discussed above for conventional Ziegler-Natta catalysts. The resulting system is composed of catalysts which have a singular activity and selectivity. For this reason, metallocene catalyst systems are often referred to as “single site” owing to their homogeneous nature. Polymers produced by such systems are often referred to as single site resins in the art.
With the advent of coordination catalysts for ethylene polymerization, the degree of long-chain branching in an ethylene polymer was substantially decreased, both for the traditional Ziegler-Natta ethylene polymers and the newer metallocene catalyzed ethylene polymers. Both, particularly the metallocene copolymers, are linear polymers or substantially linear polymers with a limited level of long chain branching. These polymers are relatively difficult to melt process when the molecular weight distribution is less than about 3.5. Thus, a dilemma appears to exist—polymers with a broad molecular weight distribution are easier to process but may lack desirable solid state attributes otherwise available from metallocene catalyzed copolymers. On the contrary, linear or substantially linear polymers catalyzed by a metallocene catalyst have desirable physical properties in the solid state but may nevertheless lack the desired processability when in the melt.
The introduction of long chain branches into substantially linear olefin copolymers has been observed to improve processing characteristics of the polymers. Such has been done using metallocene-catalyzed polymers where significant numbers of olefinically unsaturated chain ends are produced during the polymerization reaction. The olefinically unsaturated polymer chains can become “macromonomers” or “macromers” and can be re-inserted with other copolymerizable monomers to form the branched copolymers. However, the levels of long chain branching attainable with known methods thus far are not as high as those observed in LDPE made by free radical polymerization. Another limitation with the existing polyethylene compositions is that, although the processability, ease of melt processing, or increase in shear-thinning properties can be improved with the introduction of long chain branching in the polymers, the molecular weight distribution tends to increase with increased branching.
So far, improved processability may be achieved by blending different molecular weight polyethylene copolymer components or introducing a limited level of branching into polyethylene polymers. Accordingly, it generally has been thought that the advantages of the narrow molecular weight distribution made possible by metallocene catalysis needed to be sacrificed, at least in part, if improved processibilty is sought. For these reasons, there is a need for a polymerization process which could produce a polymer with melt processing characteristics similar to or better than LDPE while exhibiting solid state properties comparable to a metallocene-catalyzed polymer.